It's an incremental progression through the graceful steps from cherubic fledgling to gangly student to swan-like pre-professional dancer, but watching that development unfold onstage is one of the most interesting parts of the San Francisco Ballet School's annual showcase performances, taking place this week at the Novellus Theater at Yerba Buena Center for the Arts.

Nearly 400 aspiring dancers study at the San Francisco Ballet School, from the youngsters of the beginning levels, who will show simple classroom exercises onstage, to those in the advanced levels, who will dance excerpts from Helgi Tomasson's "Ballet d'Isoline" and "Beads of Memory," as well as pieces by faculty member Parrish Maynard and Ballet corps member Myles Thatcher. The yearly showcase is a chance to glimpse the dancers who may one day grace the War Memorial Opera House stage, indeed, who may have already appeared with the company in children's roles in ballets like "Nutcracker" or "Giselle."

"Level 2 is the youngest group to perform and they'll do nice, simple tendus, little jump combinations, to show the beginnings of a clean classical technique," says Pascale Leroy, a former soloist with the Ballet who joined the school's faculty in 1993. "Level 3 will add a little bit more movement. In Level 4, they will dance in more complex patterns. Level 5 demonstrates some pointe work, and Level 6 adds more movement still with the pointe work."

Each two- or three-minute segment captures the essence of what the students have learned that year, but even more important, says Leroy, allows them not just to show off but also to demonstrate control.

"For a lot of parents, especially of the younger ones, they may not be familiar with why does it take so long, so many levels to make a dancer," says Leroy. "So in the showcase, they see as it's unfolding, the idea of building a house with a good foundation. Then they see Level 7 and 8 and the trainees, who are almost professional, doing company repertoire and it makes sense to the parents, why the commitment, why the students must study more hours every year. And for the kids, they watch the more advanced levels; and it inspires them and gives them a sense of what they are going to do if they keep working hard."

Readying dozens of excited youngsters for the showcase is a lengthy process that begins in earnest, says faculty member Shannon Bresnahan, in January or February.

Though Bresnahan teaches the more advanced students as well, for this year's showcase she's choreographing the demonstration for the Level 5 girls, who range in age from 11 to 13.

"Level 5 is the first year that they do anything performing on pointe, so they're so excited," she says. "In a way, I start preparing them in September when they're just beginning, because I know that we have to build these ladies up to be able to perform at the end of the year on pointe. It's such a crossroads for them."

Bresnahan, who's taught at the school since 1997, is a compact dynamo of enthusiasm and warmth, frequently referring to her students with endearing nicknames - little ponies, adorable puddings.

Although the class-by-class demonstrations are devised by different faculty members, they are tied together thematically, often through the music, which this year is from the classical ballet "Le Corsaire."

"I have to do something that makes them feel good, but it also has to be presentable and appropriate for the audience," Bresnahan says. "Because they're just beginning on pointe, it has to be something they can actually do, but it has to be interesting, because the steps are very remedial. But still, they're so excited, like little puppies!"

Bresnahan, who has a reputation among her former students for being both demanding and encouraging, has seen scores of dancers climb from one level to the next and then into professional companies.

"Of course they get frustrated at times because ballet is a very difficult art form. I tell them, 'Well, honey, you're only 12 and it takes a long time to get strong,' " says Bresnahan. "But that's what I love, I love teaching and I love the process. It's like tending a garden and watching a little rose bloom more and more. It's thrilling to be the one who puts the fertilizer in the soil."

Even with all the focused determination and drive that the young dancers must have, there are still reminders, though, that many of these professional hopefuls are still kids.

"In the middle of all of this, here I am trying to get them on pointe and ready to perform, and one of them raises her hand and says, 'Miss Shannon? I just lost a tooth!' " Bresnahan recounts with a laugh.